The foundation of microkinetic analysis over 10 years ago has drastically changed the way of parametrization of rates of surface-catalyzed reactions. In the initial stages of development, some parameters arose from experimental data whereas others were fitted to experimental data. Semiempirical and first principles quantum mechanical and statistical mechanics simulations nowadays are powerful tools in estimating kinetic parameters. However, even in best cases, some tuning of parameters is typically necessary for quantitative model predictions. During this process, parameters of reaction mechanisms may violate thermodynamics. Here we review thermodynamic constraints of reaction networks and derive expressions applicable to surface reactions. We present three examples of ethylene hydrogenation, ammonia synthesis, and hydrogen oxidation to assess the thermodynamic validity of literature mechanisms. Various methods to ensure thermodynamic consistency are discussed and demonstrated with a specific example of H2 oxidation on Pt. The use of semiempirical techniques, such as the bond order conservation (BOC), known also as unity bond index-quadratic exponential potential (UBI-QEP) or Polanyi free energy relations, and first principles density functional theory (DFT), in conjunction with fitting of experimental data is discussed. Finally, mechanisms compatible with Surface CHEMKIN are proposed.

A comprehensive surface reaction mechanism on Pt is presented that is capable of describing CO oxidation, H 2 oxidation, water-gas shift (WGS), preferential oxidation (PROX) of CO, and the promoting role of H 2 O on CO oxidation reasonably well. This mechanism consists of a literature CO oxidation model, a surface reaction mechanism for H 2 oxidation on Pt developed here, and coupling reactions between the CO and H 2 chemistries included for the first time. Thermodynamic consistency, which is shown to be essential for WGS, is ensured in all steps of the entire mechanism. The CO-H 2 coupling via the CO + OH reaction, which may involve direct CO 2 formation, CO* + OH* T CO 2 * + H*, as well as an indirect pathway via the carboxyl intermediate, is explored. It is shown that this coupling plays a significant role in capturing the promoting effect of H 2 O on the CO oxidation-temperature-programmed reaction experiments at low temperatures as well as the overall speed of the WGS and PROX reactions. With the parameters used here, the direct path dominates in the water-promoted low-temperature CO oxidation, whereas the indirect path is more or equally important in the WGS and PROX reactions, depending on the operating conditions. Finally, it is found that the facilitation of the disproportionation of H 2 O*, H 2 O* + O* f 2OH*, via hydrogen bonding, is a possible mechanism for low-temperature CO oxidation via the direct path.

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